[RFC PATCH 5/5] use next syscall data to predefine the file descriptor value

[Nadia.Derbey-6ktuUTfB/bM@public.gmane.org]

[-- Attachment #1: fileopen_use_next_syscall_data.patch --]
[-- Type: text/plain, Size: 2589 bytes --]

[PATCH 05/05]

This patch uses the value written into the next_syscall_data proc file
as a target file descriptor for the next file to be opened.

This makes it easy to restart a process with the same fds as the ones it was
using during the checkpoint phase, instead of 1. opening the file, 2. dup2'ing
the open file descriptor.

The following syscalls are impacted if next_syscall_data is set:
. open()
. openat()

Signed-off-by: Nadia Derbey <Nadia.Derbey-6ktuUTfB/bM@public.gmane.org>

---
 fs/open.c |   58 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 57 insertions(+), 1 deletion(-)

Index: linux-2.6.26-rc5-mm3/fs/open.c
===================================================================
--- linux-2.6.26-rc5-mm3.orig/fs/open.c	2008-06-25 17:11:06.000000000 +0200
+++ linux-2.6.26-rc5-mm3/fs/open.c	2008-07-01 17:51:53.000000000 +0200
@@ -967,6 +967,55 @@ struct file *dentry_open(struct dentry *
 EXPORT_SYMBOL(dentry_open);
 
 /*
+ * Marks a given file descriptor entry as busy (should not be busy when this
+ * routine is called.
+ *
+ * files->next_fd is not updated: this lets the potentially created hole be
+ * filled up on next calls to get_unused_fd_flags.
+ *
+ * Returns the specified fd if successful, -errno else.
+ */
+static int get_predefined_fd_flags(int fd, int flags)
+{
+	struct files_struct *files = current->files;
+	int error;
+	struct fdtable *fdt;
+
+	error = -EINVAL;
+	if (fd < 0)
+		goto out;
+
+	error = -EMFILE;
+	if (fd >= current->signal->rlim[RLIMIT_NOFILE].rlim_cur)
+		goto out;
+
+	spin_lock(&files->file_lock);
+	fdt = files_fdtable(files);
+
+	error = expand_files(files, fd);
+	if (error < 0)
+		goto out_unlock;
+
+	error = -EBUSY;
+	if (FD_ISSET(fd, fdt->open_fds))
+		goto out_unlock;
+
+	FD_SET(fd, fdt->open_fds);
+	if (flags & O_CLOEXEC)
+		FD_SET(fd, fdt->close_on_exec);
+	else
+		FD_CLR(fd, fdt->close_on_exec);
+
+	BUG_ON(fdt->fd[fd] != NULL);
+
+	error = fd;
+out_unlock:
+	spin_unlock(&files->file_lock);
+out:
+	return error;
+}
+
+/*
  * Find an empty file descriptor entry, and mark it busy.
  */
 int get_unused_fd_flags(int flags)
@@ -1081,7 +1130,14 @@ long do_sys_open(int dfd, const char __u
 	int fd = PTR_ERR(tmp);
 
 	if (!IS_ERR(tmp)) {
-		fd = get_unused_fd_flags(flags);
+		if (next_data_set(current)) {
+			int next_fd = get_next_data(current);
+
+			fd = get_predefined_fd_flags(next_fd, flags);
+			reset_next_syscall_data(current);
+		} else
+			fd = get_unused_fd_flags(flags);
+
 		if (fd >= 0) {
 			struct file *f = do_filp_open(dfd, tmp, flags, mode);
 			if (IS_ERR(f)) {

--